1. Field of the Invention
This invention relates to articles molded from moisture shrinkable resins. More particularly, this invention concerns a molded article which is in the form of a film.
2. Description of the Prior Arts
Among shrinkable molded articles, heat shrinkable synthetic resin films have found wide application for packing purposes.
Heat shrinkable synthetic films are molded from a resin comprising such polymers as poly(vinyl chloride), poly(vinylidene chloride), polyethylene, polypropylene, polybutadiene, polyamide. Heat shrinkable synthetic resin films are used for wrapping vegetables, fruits, meats, processed foods, for lump packing fine parts, notions, for seal packing electronic devices, aerozol products and for the like.
Heat shrinkable films, however, suffered from a drawback that they could not be used for packing articles whose qualities would be degraded at elevated temperatures, since the heat shrinkable films shrink by application of heat. Also, they required heating equipments for their shrinkage.
To obviate such disadvantages, U.S. Pat. No. 4,839,450, G. F. Fanta et al. disclose that starch-poly(methyl acrylate) graft copolymers prepared from hot water-soluble modified starches can be processed to produce moisture-shrinkable films. As formulations of the copolymer and a plasticizer are shaped into films, they are stretched to induce biaxial orientation of the shaped products. The resultant films are dimentionally stable at low moderate relative humidities, but they shrink when exposed to relative humidities approaching 100% at ambient temperature.
Since the moisture shrinkable film is molded from a resin containing .alpha.-starch as its structure, the film suffered from a drawback that, even though its tensile strength in the dry state before shrinkage was satisfactory, the film would undergo a significant drop in tensile strength during its shrinkage by moisture absorption. When such a film is used for wrapping articles and exposed to moisture, the film would rupture during its shrinkage due to its deteriorated tensile strength. The film shrunk by moisture absorption would also experience a further decrease in tensile strength at the position comprising the .alpha.-starch component parts through aging. Therefore, a package wrapped with such a moisture shrinkable film may gradually become ruptured after prolonged storage.